Spdk/doc/accel_fw.md

# Acceleration Framework {#accel_fw}

SPDK provides a framework for abstracting general acceleration capabilities
that can be implemented through plug-in modules and low-level libraries. These
plug-in modules include support for hardware acceleration engines such as
the Intel(R) I/O Acceleration Technology (IOAT) engine and the Intel(R) Data
Streaming Accelerator (DSA) engine. Additionally, a software plug-in module
exists to enable use of the framework in environments without hardware
acceleration capabilities. ISA/L is used for optimized CRC32C calculation within
the software module.

The framework includes an API for getting the current capabilities of the
selected module. See [`spdk_accel_get_capabilities`](https://spdk.io/doc/accel__engine_8h.html) for more details.
For the software module, all capabilities will be reported as supported. For the hardware modules, only functions
accelerated by hardware will be reported however any function can still be called, it will just be backed by
software if it is not reported as a supported capability.

## Acceleration Framework Functions {#accel_functions}

Functions implemented via the framework can be found in the DoxyGen documentation of the
framework public header file here [accel_engine.h](https://spdk.io/doc/accel__engine_8h.html)

## Acceleration Framework Design Considerations {#accel_dc}

The general interface is defined by `/include/accel_engine.h` and implemented
in `/lib/accel`.  These functions may be called by an SPDK application and in
most cases, except where otherwise documented, are asynchronous and follow the
standard SPDK model for callbacks with a callback argument.

If the acceleration framework is started without initializing a hardware module,
optimized software implementations of the functions will back the public API.
Additionally, if any hardware module does not support a specific function and that
hardware module is initialized, the specific function will fallback to a software
optimized implementation.  For example, IOAT does not support the dualcast function
in hardware but if the IOAT module has been initialized and the public dualcast API
is called, it will actually be done via software behind the scenes.

## Acceleration Low Level Libraries {#accel_libs}

Low level libraries provide only the most basic functions that are specific to
the hardware. Low level libraries are located in the '/lib' directory with the
exception of the software implementation which is implemented as part of the
framework itself. The software low level library does not expose a public API.
Applications may choose to interact directly with a low level library if there are
specific needs/considerations not met via accessing the library through the
framework/module. Note that when using the low level libraries directly, the
framework abstracted interface is bypassed as the application will call the public
functions exposed by the individual low level libraries. Thus, code written this
way needs to be certain that the underlying hardware exists everywhere that it runs.

The low level library for IOAT is located in `/lib/ioat`.  The low level library
for DSA is in `/lib/idxd` (IDXD stands for Intel(R) Data Acceleration Driver).
In `/lib/idxd` folder, SPDK supports to leverage both user space and kernel space driver
to drive DSA devices. And the following describes each usage scenario:

Leveraging user space idxd driver: The DSA devices are managed by the user space
driver in a dedicated SPDK process, then the device cannot be shared by another
process. The benefit of this usage is no kernel dependency.

Leveraging kernel space driver: The DSA devices are managed by kernel
space drivers. And the Work queues inside the DSA device can be shared among
different processes. Naturally, it can be used in cloud native scenario. The drawback of
this usage is the kernel dependency, i.e., idxd driver must be supported and loaded
in the kernel.

## Acceleration Plug-In Modules {#accel_modules}

Plug-in modules depend on low level libraries to interact with the hardware and
add additional functionality such as queueing during busy conditions or flow
control in some cases. The framework in turn depends on the modules to provide
the complete implementation of the acceleration component. A module must be
selected via startup RPC when the application is started. Otherwise, if no startup
RPC is provided, the framework is available and will use the software plug-in module.

### IOAT Module {#accel_ioat}

To use the IOAT engine, use the RPC [`ioat_scan_accel_engine`](https://spdk.io/doc/jsonrpc.html) before starting the application.

### IDXD Module {#accel_idxd}

To use the DSA engine, use the RPC
[`idxd_scan_accel_engine`](https://spdk.io/doc/jsonrpc.html). By default, this
will attempt to load the SPDK user-space idxd driver. To use the built-in
kernel driver on Linux, add the `-k` parameter. See the next section for
details on using the kernel driver.

The DSA hardware supports a limited queue depth and channels. This means that
only a limited number of `spdk_thread`s will be able to acquire a channel.
Design software to deal with the inability to get a channel.

### How to use kernel idxd driver {#accel_idxd_kernel}

There are several dependencies to leverage the Linux idxd driver for driving DSA devices.

1 Linux kernel support: You need to have a Linux kernel with the `idxd` driver
loaded. Futher, add the following command line options to the kernel boot
commands:

```bash
intel_iommu=on,sm_on
```

2 User library dependency: Users need to install the `accel-config` library.
This is often packaged, but the source is available on
[GitHub](https://github.com/intel/idxd-config). After the library is installed,
users can use the `accel-config` command to configure the work queues(WQs) of
the idxd devices managed by the kernel with the following steps:

```bash
accel-config disable-wq dsa0/wq0.1
accel-config disable-device dsa0
accel-config config-wq --group-id=0 --mode=dedicated --wq-size=128 --type=user --name="MyApp1"
 --priority=10 --block-on-fault=1 dsa0/wq0.1
accel-config config-engine dsa0/engine0.0 --group-id=0
accel-config config-engine dsa0/engine0.1 --group-id=0
accel-config config-engine dsa0/engine0.2 --group-id=0
accel-config config-engine dsa0/engine0.3 --group-id=0
accel-config enable-device dsa0
accel-config enable-wq dsa0/wq0.1
```

DSA can be configured in many ways, but the above configuration is needed for use with SPDK.

### Software Module {#accel_sw}

The software module is enabled by default. If no hardware engine is explicitly
enabled via startup RPC as discussed earlier, the software module will use ISA-L
if available for functions such as CRC32C. Otherwise, standard glibc calls are
used to back the framework API.